GlobalISel: Implement lower for G_FCOPYSIGN

GlobalISel: Implement lower for G_FCOPYSIGN

In SelectionDAG AMDGPU treated these as legal, but this was mostly
because the bitcasts required for FP types were painful. Theoretically
the bitpattern should eventually match to bfi, so don't bother trying
to get the patterns to import.

llvm-svn: 365583